Many methods for automatic duplex printing in xerographic processors have been devised. There is the classical two pass method employed in the Xerox 4000 and 9400 reproduction machines. That is, after the first side of a copy sheet is imaged and fused, the sheets are collected in a duplex tray. After the last sheet in a set has been received in the duplex tray, the sheets are again passed through the processor. This time an image is transferred and fused onto the opposite side of each copy sheet having an image on a first side.
In the Xerox 9700 machine, the copy sheets also pass through the processor twice. However, they are not collected in a duplex tray. After the first image has been transferred and fused, the sheets pass through a stop and reverse mechanism (inverter). Then the sheets join in an interleafing fashion the stream of copy sheets to receive an image on the opposite side.
There are some disadvantages with these systems, in particular for a given image throughput rate. For example, two passes through the fuser require more energy, and the fuser needs to operate at twice the speed. During the first pass through the fuser, the paper loses 50 percent of its moisture. This curls the paper (especially thick stock) and makes the second pass for duplexing hazardous. Paper picks up oil on the first pass through the fuser, sometimes leading to image deletions on the second image and oil deposits on the photoreceptor. Jam rates during two-pass duplex operation are much greater than for simplex operation. In the first place, in a two-pass duplex system, the paper path is usually very long, and the paper has to negotiate all obstacles twice. Excessive paper curl is not only troublesome in the processor but also extremely difficult to handle in output stackers and finishing devices.
In other prior art systems such as in U.S. Pat. No. 4,095,979, means are shown for "immediate" or single pass duplex copying of forming first and second images sequentially on a photoreceptor. The first image is transferred from the photoreceptor to the first side of a copy sheet. Then the sheet is stripped off the photoreceptor, inverted while the first image remains unfixed, and then the second image is transferred to the second side of the copy sheet. Both images are then fixed onto the copy sheet in a suitable fuser. This type of system can be described as a "single pass" to the fuser.
Other single pass duplex printing methods use intermediate image carriers (belt or drum). The first and second images are sequentially formed on a photoreceptor. The first image is transferred to an intermediate image carrier. The copy sheet is then passed between the photoreceptor and the intermediate iamge carrier, simultaneously receiving first and second images.
One of the problems with the prior art or single pass duplex systems is in conveying the duplex copy sheet to the fuser. In particular, the copy sheet with the two unfused images on opposite sides, must be transported from its second stripping point (after the second side transfer) into the fuser. This cannot be done with a conventional photoreceptor fuser transport since the transport would contact one of the sides of the copy sheet and smear one unfused toner image. Also, to avoid the lead edge of the sheet from dropping in the path between transfer and fuser must be very short, and the fuser must be very close to the photoreceptor. This creates problems in mechanical mounting, problems of heating of the photoreceptor and problems of contamination of the photoreceptor with fuser release materials, e.g. silicone oil vapor.
In addition there is the problem of the uncontrollable velocities of sheets passing through roll fusers. There is an obvious need to accurately match the velocity of the copy sheet transport with the velocities of the photoreceptor to prevent "skips" and "smears" during transfer. Furthermore, for high resolution digital printing, excessive instantaneous photoreceptor velocity variations (jitter) cannot be tolerated. Fuser rolls are notorious for creating such variations. Even in conventional copiers it is preferable to keep the fuser rolls one sheet length away from the transfer zone. For these reasons it is therefore desirable to thermally insulate and mechnically isolate the photoreceptor transfer zones from the hot fuser rolls.
The duplex methods above only utilize one photoreceptor. Other systems, e.g. U.S. Pat. No. 3,580,070 and 3,775,102 deal with "single pass duplex" methods employing two photoreceptors and two exposure systems. First images are deposited on one photoreceptor and second images are deposited on the other photoreceptor. These systems are considered the ultimate duplex throughput systems since they produce twice the number of images of "two pass duplex" systems at equal process speed. These single pass duplex systems, however, generally require web paper feed in which the copy is spooled up on a roll or cut into individual sheets after fusing. This unfortunately, introduces additional components and complexity into the system. It is, therefore, also desirable to provide a single pass duplex system having a discrete copy sheet feed system rather than a web paper feed system.
It is therefore an object of the present invention to provide a paper handling system adapted to transport cut sheets of a variety of dimensions through a dual photoreceptor processor for direct duplex printing.
It is a further object to provide a two photoreceptor, single pass duplex system having a relatively short paper path between the fuser and the second transfer station.
Still another object of the present invention is to provide a prefuser copy tansport device acting as a heat shield between the fuser and the photoreceptor.
Further advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.
Briefly, the present invention is concerned with a two photoreceptor, single pass duplex reproduction system having a heat insulating prefuser transport device and first and second transfer stations. The prefuser transport device is disposed between the fuser and the transfer stations. In particular, the prefuser transport is a pair of cold, toner compacting rolls adjacent the second transfer station for immediate pick up of a copy sheet supporting unfused images on both sides. The compacting rolls tack the unfused images to the copy sheet. The compacting rolls also insulate the photoreceptor from the heat of the fuser and convey the copy sheet immediately to the fuser. The fuser permanently fixes the images onto the copy sheet in one fuser operation. In a preferred embodiment, the fuser rolls operate at a slightly lower peripheral velocity than the compacting rolls. Also, because of the tacking of the image by the cold rolls, the fuser rolls may operate at a relatively lower temperature or pressure than normally.
Other objects and advantages of the present invention become apparent upon reading the following detailed description and upon reference to the drawing which is a schematic side view of the copy sheet conveying apparatus in accordance with the present invention.